Backdrop: Fix stale body by removing unnecessary default

Created by: weaverryan

Hi there!

I'm using Turbo, where the entire body element is swapped out at times. This causes a bug with Backdrop, as it stores the original body element in the Default object. This is the flow:

1. Open a Modal / Backdrop
2. Navigate to another page (i.e. swap the body for a new body Element)
3. Open another Modal / Backdrop

On (3), the Backdrop will append itself to the original body Element from (1), not the new body element.

Also, the default is unnecessary: in _getConfig(), if config.rootElement is falsy, then it already defaults to document.body (which is why I didn't add any tests: there are tests already to make sure the rootElement can be configured):

https://github.com/twbs/bootstrap/blob/136665903b50988002380f512342dcb584e1bed4/js/src/util/backdrop.js#L93

Please let me know if any tweaks are needed.

Thanks!